compile error: ifstream::open only accepts string values in quotes "" and not string variables

Question

Does open function have some sort of restriction as to what kind of string value is passed in?

ifstream file;
string filename = "output.txt";
file.open(filename);

I tried to pass a string value with a string variable, but when it tries to compile, the result is...

agent.cpp:2:20: error: ofstream: No such file or directory
agent.cpp: In function ‘std::string readingline(std::string)’:
agent.cpp:11: error: aggregate ‘std::ifstream file’ has incomplete type and cannot be     defined
agent.cpp: In function ‘int main()’:
agent.cpp:44: error: aggregate ‘std::ofstream writefile’ has incomplete type and cannot be defined

On the other hand, when I just pass a string value in quotes like "filename.txt" , it compile fine and runs fine.

ifstream file;
file.open("output.txt");

Why is this the case?

Is there a way to solve this problem?

See [Why does (i|o)fstream take a const char* parameter for a file name?](http://stackoverflow.com/questions/5972151/why-does-iofstream-take-a-const-char-parameter-for-a-file-name) — Seth Carnegie, Dec 18 '11 at 03:58

score 5 · Accepted Answer · answered Dec 18 '11 at 03:52

5

Sadly, this is how the constructor and open of std::(i|o)fstream are defined by the standard. Use file.open(filename.c_str()).

Some standard libraries provide an extension that allows std::string as a parameter, e.g. Visual Studio.

answered Dec 18 '11 at 03:52

Xeo

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Thank you Xeo. Using `file.open(filename.c_str())` worked out. – Jason Kim Dec 18 '11 at 04:07
1

It is not an extension anymore, but in compliance with the C++11 standard. – Bo Persson Dec 18 '11 at 10:13
@Bo: Wait, we got those in C++11? I must have missed that. *goes to skim through the standard* – Xeo Dec 18 '11 at 16:30
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I suppose the reason why `const char*` was preferred over `std::string` is that filenames, by (POSIX) definition can never contain embedded nulls. Saying `file.open(fname.c_str())` makes that explicit to the programmer – sehe Dec 18 '11 at 19:06

score 1 · Answer 2 · edited Dec 18 '11 at 19:41

1

I think your error messages may be unrelated to the code in question, but open takes a C-style const char* and not a C++ string. You'll need to use filename.c_str() in the call to make it work correctly.

edited Dec 18 '11 at 19:41

Xeo

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answered Dec 18 '11 at 03:51

Mark B

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Thanks Mark B for the suggestion. – Jason Kim Dec 18 '11 at 04:07

score 1 · Answer 3 · answered Dec 18 '11 at 03:52

I got the problem to go away by including fstream and passing filename.c_str() instead of just filename.

The message about an incomplete type is because you are missing a header (probably anyway, you didn't show a full example).

And open takes a c-style string, not the string class.

compile error: ifstream::open only accepts string values in quotes "" and not string variables

3 Answers3